in many optical designs, solid-glass optical components are used for reflection at their faces to provide redirection of optical imaging therethrough. However, the use of solid-glass optical components may introduce a large amount of weight into a design, rendering the design unusable. For example, various optical systems are used in conjunction with headgear of aviation navigators. Such components may include combiner elements utilized in display systems of aviation headgear. The combiner elements may be prismatic with two parallel flat surfaces for internal reflection of light therethrough. An optical system constructed of glass components may result in headgear that is too heavy to serve its intended purpose.
As is known to one skilled in the art, the use of plastic to produce light weight optical elements can solve the problem associated with the weight of glass components. The surfaces of such plastic optical elements can be machined to many various geometric surfaces, including optically flat surfaces. One method of such machining is diamond turning as described in U.S. Pat. No. 4,852,436 to Benjamin or as is generally known to one skilled in the art.
Although it is possible to diamond turn surfaces of plastic optical elements, it is expensive and difficult because of the softness of the plastic material and because of other physical characteristics of the material. Machining, such as diamond turning, does not produce a completely smooth surface. The diamond turning, for example, produces an optically flat surface on a plastic optical element which has many ridges thereon. Although these ridges are minimal, about 50 Angstroms from peak to trough, in high performance optical systems utilizing short wavelengths, such ridges can create a haze from diffraction or light scattering due to the microscopic defects or ridges as light passes through the optical element. Such haze is unacceptable when the optical element is used in vision-type systems. In order to reduce the diffraction and haze, it is necessary that the diamond turned surfaces be smoothed or be formed as near to a perfectly optically flat surface as possible so that such diffraction or light scattering due to the ridges is eliminated.
in addition, diamond turning may not produce a surface with ridges thereon, wherein the ridges are of the same height. The surface may also not fall in a perfectly flat plane as the edges of the surface may either be above or below a plane established by the center of the flat surface because of movement of the machining tool or because of the softness of the plastic material. Such differences of various locations of the flat surface produce internal reflection problems when dealing with short wavelengths. Such reflection defects become additive when you consider that both parallel flat surfaces may have such dimensional defects.
In co-pending application, U.S. Ser. No. 07/996,283 entitled "Spin Deposition of a Nonconformal Coating Surface to Machined Optical Asphere Surfaces," a method is shown for spin depositing a coating material on a plastic optical element so as to produce a nonconformal surface on a machined surface of an optical element to eliminate haze effects. However, such a spin deposition method if used to create an optically flat surface, would create a thickness gradient of the material coating on a machined flat surface which would be greater toward the edges of the machined surface as opposed to the geometric center. Such a thickness gradient would produce optical system problems. There exists a need for a method to replicate optically flat surfaces in order to eliminate the diffractional haze without producing other unwanted effects such as varied thickness over the optically flat surface in order to provide for near perfect internal reflection through an optical element, such as a combiner.